Advances in electrical energy technology are frequently impeded by a lack of appropriate means for electrical energy storage (EES). For example, renewable energy sources such as wind and solar could meet a significant proportion of the world's electricity needs. Yet these sources remain largely untapped for want of viable EES capacity. In the same way, large-scale EES will be critical to the success of initiatives to modernize the national electric power grid.
Rechargeable batteries are a proven means for EES but established battery chemistries are poorly suited for large-scale applications. They store energy in the form of solid reactants that form part of the battery electrodes. Furthermore, the electrodes must undergo physical and chemical changes each time the battery is charged and discharged. These changes impose limits not only on the cycle life but also on the maximum power (e.g., in kilowatts) that can be delivered.
Redox Flow Batteries (RFBs) are rechargeable systems in which the electrochemical reactants are dissolved in liquid electrolytes. The electrolytes, which are stored in external tanks, are pumped through a stack of reaction cells where electrical energy is alternately converted to and extracted from chemical energy in the reactants by way of reduction and oxidation reactions.
Over time the reaction stoichiometry (i.e., the stoichiometric proportions) of the two reactants may deviate from a desired relationship. When such a deviation occurs, the RFB electrolytes are said to be “imbalanced”. Prior art methods for rebalancing the electrolytes substantially degrade system efficiency, either by consuming too much energy or by releasing contaminants into the RFB electrolytes that cause additional imbalance. There remains a need for methods of rebalancing RFB electrolytes while minimizing the release of harmful contaminants.